Fix i386 exp missing underflows (bug 18961).

On i386, the double version of exp can miss underflow exceptions if
the result is in the subnormal range for double but the last 11 bits
of the 64-bit extended-precision mantissa happen to be zero.  This
patch forces the exception in a similar way to previous fixes.

As with the exp2 fixes, the expf changes may in fact not be needed to
ensure underflow exceptions, but are included for consistency and to
fix the exp part of bug 18875 by ensuring that excess range and
precision is removed from underflowing return values.

Tested for x86_64 and x86.

	[BZ #18875]
	[BZ #18961]
	* sysdeps/i386/fpu/e_exp.S (dbl_min): New object.
	(MO): New macro.
	(__ieee754_exp): For small results, force underflow exception and
	remove excess range and precision from return value.
	(__exp_finite): Likewise.
	* sysdeps/i386/fpu/e_expf.S (flt_min): New object.
	(MO): New macro.
	(__ieee754_expf): For small results, force underflow exception and
	remove excess range and precision from return value.
	(__expf_finite): Likewise.
	* math/auto-libm-test-in: Add more tests of exp.
	* math/auto-libm-test-out: Regenerated.